Coiled steel tubing finds a number of uses in oil well operations. For example, it is used in running wireline cable down hole with well tools, such as logging tools and perforating tools. Such tubing is also used in the workover of wells, to deliver various chemicals.
Steel coiled tubing is capable of being spooled because the steel used in the product exhibits high ductility (i.e. the ability to plastically deform without failure). The spooling operation is commonly conducted while the tube is under high internal pressure which introduces combined load effects. Unfortunately, repeated spooling and use causes fatigue damage and the steel coiled tubing can suddenly fracture and fail. The hazards of the operation and the high personal and economic cost of failure in down time in fishing operations forces the product to be retired after relatively few trips into a well. The cross section of steel tubing expands during repeated use, causes reduced wall thickness and results in lower pressure allowables and higher bending strains.
It is desirable to provide a non-steel coil tubing which is capable of being spooled and which does not suffer from the defects of steel tubing.
Present operations using coiled tubing for logging operations utilize a wireline cable inserted within the bore of the coiled tubing or when real time data is not required, a logging tool with downhole memory is used. When real time data is required, a dedicated reel or coiled tubing is used with the wireline permanently installed in the tubing. This takes up substantial cross-sectional space within the tubing and thus renders the coiled tubing unsuitable for other operations requiring a flow path or open bore through the tubing. Conducting fluids from the surface to a downhole location through the tubing may be used to provide means for treating formations or for operating a mud motor to drill through the formations. In addition, it may be desirable to pump devices through the coiled tubing bore to a downhole location for various operations. In any event, an open bore within the coiled tubing is essential for many operations and for this reason it is desirable to not have electrical conductors or the like positioned within the open bore portion of the tubing.